Process for forming chromate coatings on aluminum



United States Patent 3,347,713 PROCESS FOR FORMING CHROMATE (IOATINGS 0NALUMINUM Herman J. Lodeesen, Clawson, and Wiiliarn S. Russell,

Warren, Mich, assignors to Hooker Chemical Corporation, Niagara Falls,N.Y., a corporation of New York No Drawing. Filed Feb. 3, 1964, Ser. No.342,275

16 Claims. (Cl. 148-62) ABSTRACT OF THE DECLOSURE An aqueous acidicsolution useful for forming a protective coating on aluminum or aluminumalloy surfaces, which contains, in percent W./v. 0.05 to 1% CrO 0.16 to2.7% fluoride ions, 0.01 to.0.4% tungsten and 0.22 to 3.2% Al(F)complex. Preferably, this solution has a pH Within the range of about1.1 to 2.3 and the protective coating is formed on the aluminum andaluminum alloy surfaces by contacting the surface with this solution.

This invention relates to procedures and solutions for forming corrosionresistant and paint bonding coatings on the surfaces of aluminum andaluminum alloys. More particularly, this invention concerns aqueousacidic solutions of the general type which contain the hexavalentchromium ion and fluoride ions and which are improved by the inclusiontherein of a modifying constituent which improves the speed, efliciencyand ease of operation of the solution to produce adherent, corrosionresisting chromate coatings on aluminum or aluminum alloy surfaces.

The present invention relates to solutions for and methods of treatingaluminum-containing surfaces which are in the same general category asthose described in such typical United States patents as 2,276,353;2,471,909; 2,472,864; 2,796,370; 2,796,371; 2,507,956; 2,843,513;2,859,144; 2,868,679; 2,839,439; and 3,009,842. In these disclosures andother efforts to provide an effective process, it is apparent that anattempt has been made to concurrently provide a process useful for abroad range of aluminum and aluminum alloy surfaces and yet one which issimple to prepare initially and maintain in elfective coating formingcondition during use. As this art has developed, however, the trend hasbeen to add to operative solutions additional anions or cations toattain some specific objective, and by such modifications the control ofthe solutions during continuous use has been complicated and renderedmore difficult. This invention deviates from the heretofore employedapproach in providing a simplified composition which employs asubstitute for the single modifying anion ferricyanide that is disclosedand claimed in United States Patent 2,796,370.

In the commercial operation of chromate-fiuoride ferricyanide activatedsolutions for coating aluminum a number of operational difficulties havebeen encountered. Such solutions must constantly contain a certainminimum quantity of the ferricyanide ion in order to preserve thecoating rate and coating efliciency at levels which are commerciallyacceptable from the combined standpoints of quality and economy. It hasbeen observed that the ferricyanide ion is sensitive to temperaturedegradation at temperatures of above about 120125 F., and unless care isconstantly exercised to avoid exceeding this temperature, even atlocalized areas of an operating tank installation, the ferricyanidebecomes degraded and incapable of maintaining the solution inacceptanble coating-forming condition. It has also been found that suchsolutions are relatively sensitive to acidity and require attentivemaintenance of the necessary acidity in a fairly narrow range ofacidity. Moreover, chromate-fluoride ferricyanide solutions lose theircoating ability upon mere standing at room temperature without use andreplenishment of them is complicated because the ferricyanide andchromate-fluoride components must be added separately and maintained inseparate containers to avoid reaction between chromate and ferricyanide.

It is, therefore, the primary object of this invention to provide animproved chromate-fluoride solution which operates at high speed andhigh efficiency to form a corrosion-resistant, paint receptive coatingon aluminum which is capable of operating at high coating rate and highefficiency over a wide range of pH and temperature and which is suitablefor use at temperatures up to those approaching the boiling point of thesolution.

Another object of this invention is to provide an improvedchromate-fluoride aqueous acidic coating solution for aluminum which isstable upon standing at room temperature without use.

Another object of this invention is to provide a process for coatingaluminum which employs a chromate-fluoride aqueous acidic solution thatis capable of being maintained in optimum coating condition 'byreplenishing with a single replenishing material; a further object is toprovide replenishing materials for use in the improved process of thisinvention.

A further important object of this invention is to provide a process forcoating aluminum which employs a chromate-fluoride aqueous acidicsolution which is economical to use, easy to control and maintain inoptimum coating-forming condition and capable of providing adherent,corrosion-resistant, colored coatings having predetermined shades, asdesired.

The present invention is based on the discovery that aqueous acidicsolutions containing the hexavalent chromium ion and the fluoride ioncan be converted into highspeed coating-forming solutions byincorporating therein the tungsten anion so long as certain quantitiesand relative proportions of tne hexavalent chromium and tungsten anionsare maintained in the solution and the solution contains a controlledproportion of a complex aluminum fluoride ion.

It has been found that relatively minor proportions of the tungstenanion are capable of changing CrOg-fluoride ion aqueous acidic solutionfrom a slow, low-efliciency coating solution into a high speed, highefficiency solution which is easy to control and maintain in'peakcoating condition during continuous use. The solutions are predominantlychromate solutions and may contain from about 0.5 to about 10grams/liter CrO In these solutions which also contain a sufficientquantity of the fluoride ion to cause attack of the aluminum surface andresulting coating formation, it has been found that at least a minimumof about 0.1 grams/liter of tungsten is necessary in the solution inorder to obtain a significant improvement in the rate and efficiency ofcoating formation. This minimum quantity of tungsten is most effectiveto form high speed, adherent, corrosion resistant coatings on thesurfaces of aluminum and aluminum alloys when there is also present inthe solution a quantity of an aluminum fluoride complex ion equivalentto about 2.2 to about 32 grams/liter of Al(F) In the absence of such acontrolled quantity of Al(F) it has been observed that the highest speedof coating and highest efiiciency are not attained, although thetungsten anion alone, produces a markedimprovernent in the coatingability of a CrO fluoride ion activated aqueous acidic solution. Thealuminum fluoride complex ion is expressed as Al(F) because it ispresent in the operating solution as an equilibrium of AIF ions whichmay contain from 1 to 6 fluoride atoms per aluminum atom and in thesolutions of this invention this equilibrium averages out to beapproximately equivalent to the AlF ion. The expression AMP) is intendedto represent any aluminum fluoride ion and the quantities thereof referto an amount of such ion equivalent to the AIR, ion.

Coating efliciency as used herein and in the appended claims refers tothe relative quantity of coating formed to the amount of metal dissolvedfrom the surface of the metal being coated and is specifically the ratioobtained by dividing the metal loss in milligrams per square ft. ofsurface treated by the coating weight on the same area in milligrams persquare ft. Thus, as the numerical ratio decreases, the coatingefficiency increases and the lowest numbers represent the highestefliciency of coating formation. For example, a chromic acid solutioncontaining the fluoride ion as its sole activator ion dissolves betweenabout 40 and about 200 milligrams per square ft. of aluminum metal inorder to form on that surface a coating having a weight of 100milligrams per square ft.; such coating formation is relatively slow andthe coating efliciency is between 0.4 and 2.0, which is a relatively lowcoating efficiency. In comparison, the use of a typical coating solutionof this invention forms 100 milligrams per square ft. of coating whiledissolving only between about to about 33 milligrams of aluminum metalper square ft.; such coatings are formed at high speed, measured inseconds, and the coating efficiency varies between 0.15 and 0.33, whichis a relatively high efiiciency range. The improved compositions of thisinvention include, the hexavalent chromium ion, the fluoride ion, thetungsten ion and an aluminum fluoride ion and these ions may be presentin the relative proportions, expressed in percent, weight/volume, as setforth in the Formulations I and II which follow:

Formulation I Concentration,

Solution component: percent w./v. CrO 0.05-1 Fluoride ion 0.16-2.7Tungsten anion as tungsten 0.01-0.4 Al(F) complex 0.22-3.2'

A preferred formulation for the purposes of this invention is set forthbelow as Formulation II.

Formulation II Concentration,

Solution component: percent w./v. CrO 0.20.5 Fluoride ion 0.16-1.6Tungsten anion as tungsten 0.030.1 Al(F) complex 022-19 The heaxavalentchromium ion may be supplied as chromic acid or one or more of the watersoluble salts thereof, including the sodium, potassium or ammoniumchromates or dichromates or admixtures thereof with chromic acid and itssalts.

The fluoride ion may be supplied by any fluorine-containing compoundwhich is capable of ionizing in the aqueous acidic solutions of thisinvention to provide the fluoride ion, such as hydrofluoric acid,fluosilicic acid, or fluoboric acid and the sodium, potassium andammonium salts thereof. The aluminum fluoride ion may be supplied assuch or may be formed in the bath as a complex from free aluminum andfluoride ion. For this purpose the fluoride may be present as HF, HBF orH SiF The tungsten ion may be supplied in the solution of this inventionby adding thereto any ionizable tungsten compound such as tungstic acid,or its sodium or potassium or ammonium salts, which will give thetungsten anion when oxidized by chromic acid or the salts thereof.

The solutions of this invention may be used to form coatings on thesurfaces of aluminum or aluminum alloys by dipping, brushing, orspraying the solution on the sur- 4 face after conventional cleaningprocedures have been employed to free the surface of oil, grease,oxideor the like. The solutions may alsobe applied to form coatings byatomizing the solution on the surface in a heated condition. Generallystated, this atomization application procedure comprises the steps ofpreliminarily heating the aluminum or aluminum alloy surface to becoatedto a temperature above about F. and atomizing on that heatedsurface a quantity of the coating solution sufficient to form thedesired coating but insufficient to cause the droplets of atomizedcoating solution to coalesce or puddle on the surface, the coatingresulting from the substantially instantaneous flashing of the liquidfrom the solution and each individual atomizedparticle droplet remainingsubstantially' in the locus of its original contact withthe aluminum oraluminum alloy surface. The coatings of this invention are slightlycolored and vary in appearance from iridescent to light-gold to yellowto brown, generally as the coating weight increases.

The highspeed, high efficiency coating method of this invention broadlycomprises the above enumerated steps of using the selected form ofapplication of the coating solution to the aluminum or aluminum alloysurface to pending uponthe acidity and the concentration of the.

tungsten and aluminum fluoride complex ion in the solution, it ispossible to obtain an increase in the coating rate of from 2-5 timesthat obtainedat room temperature from a solution containing a constantquantity of CrO and the fluoride ion. The coating rate-has been found toincrease much more slowly between 120 F. and F., and for practicalpurposes the coating rate is substantially uniform throughout thisrange. It is, therefore, preferred to operate the solutions of thisinvention at a temperature in the range of about 120-160 F. Highertemperatures than 160 F. may be employed, for example, F. or even up tothe boiling point of the solution, but no particular advantage isobtained by operating at temperatures higher than about 160 F.

The pH of the operating solution significantly affects the coating rateand coating efliciency of the solution being applied to the aluminum oraluminumalloy surface but excellent results are obtained over a fairlywide range of pH variations. The solutions may be operatedsatisfactorily in the pH range of about 1.1 to about 2.3 and preferablyin the range of about 1.6 to about 2.1. The pH range refers tomeasurements taken by using an electrical pH meter employing a glasselectrode and a calomel electrode by immersing the electrodes infreshportions of the solution and observing the indicated value. Theglass electrode is maintained in optimum condition by taking care torinse the electrode, immediately upon its removal from the solutionbeing tested, in a solution of five normal hydrochloric acid byinserting in that solution for two to three minutes and thereafterrinsing the electrode in pure water. Between measurements, the electrodeis kept immersed in pure water and prior to each use is checked againsta standard buffer.

The tungsten anion containing chromate-fluoride solutions of thisinvention are basically different from any of the heretofore knownchromate solutions for coating aluminum, and although the entire reasonfor this fact is not known with certainty, the evidence is clear thatthe concurrent chemical reactions occur, ionic balances are changing andresultant concentrations and relative proportions of active ingredientsmigrate toward a condition of inbalance and loss of coating-formingability. In the use of the solutions of this invention, for example, atleast the following changes are simultaneously occurring: hexavalentchromium, fluoride and tungsten anions are used in metal attack andcoating formation, aluminum is released during metal attack, remains inthe solution and forms varying aluminum fluoride complexes dependingupon pH and temperature, some hexavalent chromium is reduced totrivalent chromium which remains in the solution and at least partiallyforms complexes with fluoride, acid is consumed in coating formation andthe pH tends to rise, some of the tungsten anion and aluminum fluoridecomplexes apparently form additional stabilizing complexes and as anoverall result the relative proportions of hexavalent chromium, fluorideand tungsten tend to change.

In solutions otherwise similar except that they contain no tungstenanion, some of the above enumerated changes occur, but in each case to asubstantially different degree and with a significantly different finaleffect. For example, when using a simple CrO -fluoride ion activatedaqueous acidic solution within the herein disclosed pH range, a smallportion of the hexavalent ion decreases by becoming a part of thecoating and a large part of the hexavalent chromium is reduced totrivalent chromium and remains in the solution, some fluoride ioncomplexes with the trivalent chromium thus released in the solution,some fluoride complexes with the aluminum released during metal attackand the acid content decreases; however, the fluoride ion concentrationdecrease in the simple CrQ -fluoride ion solution is substantiallygreater than occurs in the solution which is otherwise similar exceptthat it contains the tungsten anion. As a result of the decrease influoride ion concentration and build-up of other ions the solutionsolution soon loses its ability to form a coating. In the simplechromate-fluoride solutions, the chromate-fluoride-ferricyanidesolutions, the chromatophosphate-fluoride solutions, one of theheretofore employed procedures has been to add acid to the solution,such as sulfuric, nitric or other mineral acid. The acid addition hasrestored coating ability and increased the coating rate, and this effecthas been explained to be due to a shift in the equilibrium or aluminumfluoride complexes toward those containing less fluoride, thus freeingfluoride for metal attack and coating formation. The addition of such amineral acid to the tungsten anion containing solutions of thisinvention does not effect a comparable change in the coating rate;rather the solutions operate over a wide range of pH to provideapproximately the same coating rate. In a somewhat similar fashion, ithas been observed that the solutions of this invention provide about thesame coating rate and coating efliciency over a range of temperaturefrom about 120 'to' about 160 F., whereas the ordinary effect of raisingthe temperature of an aqueous acidic metal coating solution in thisrange is to increase the coating weight proportionately. There is aslight advantage in coating rate at about 130 F.140

F. but it is not commercially significantly better than other portionsof the range between 120 F. and 160 F.

The presence in the operating solutions of this invention of foreigncations, that is, cations other than the alkali metal ions normallyintroduced together with the fluoride or tungsten ion, such as divalentor trivalent metal ions, have been found to be undesirable and todetrimentally affect the coating rate and coating efliciencies which maybe obtained by using the solutions of this invention in the manner abovedescribed. This detrimental eflFect on coating rate and coatingefiiciency is particularly noticeable in those solutions of thisinvention which contain the lower quantities of tungsten and/ orfluoride ions, and although minor quantities of such undesirable cationscould be off-set by the addition of anions such as the sulfate ornitrate anion, to thus permit the attainment of the benefits ofconcurrently using the tungsten, fluoride and aluminum fluoride complexions, it is nevertheless undesirable to introduce them into or have suchother cations present in the solutions. A particularly advantageousprocedure for maintaining the solutions of this invention in optimumoperating condition is to operate the solutions in conjunction with anion exchange unit of the type and by the procedures described in Roy A.Halversen United States Patent 2,967,- 791, issued Jan. 10, 1961, andfor the majority of installations, this method of operation ispreferred. In operating the solutions of this invention with such an ionexchange unit which employs a cation exchange resin, the solutions aremaintained free of metallic cations other than aluminum which occurs inthe solutions as aluminum flouride complex ions and these complexes arecontrolled in concentration in the solution by the ion exchange unit.

Using the method of this invention, corrosion resistant, adherentcoatings are formed on the aluminum or aluminum alloy surfaces in amatter of a few seconds, for example, about 1 to about 20 seconds and bymodifying the compositions as to tungsten anion content, fluoride ioncontent, aluminum fluoride complex ion content, and as to pH andacidity, it is possible to maintain the coating rate suificiently fastto form a coating weight on continuous strips of aluminum or aluminumalloys having a weight in the range of about 15 to about 40 milligramsper square ft. in 1 to 2 seconds of contact time.

The below given examples set forth in somewhat greater detail typicaloperating solutions and operating procedures for the purpose ofillustrating certain of the above referred to effects of variation insolution composition and operating conditions. It is to be understood,however, that these examples are presented for illustrative purposesonly and do not themselves represent the definite compositional limitsor limits on the operational steps of this invention which have beendefined hereinabove and hereinafter in the appended claims.

EXAMPLE I gold in color and had coating weights varying from about 20 toabout 50 milligrams per square ft., average, depending upon the contacttime. The coating efilciency calculated as metal loss in milligrams persquare ft. divided .by the coating weight in milligrams per square ft.averaged 0.2. This solution was maintained free of trivalent chromiumions and at the above level of aluminum fluoride complex by constantlycycling a small portion of the solution through an ion exchange resincolumn filled with Dowex 50, 20-100 mesh cation exchange resin, andafter many thousands of surface feet of'aluminum had been processedthrough the solution, the coating obtained was still similar inappearance and the coating weight and the coating efiiciency remainedsubstantially constant.

EXAMPLE II A solution was prepared containing 0.25% CrO 0.02% fluoride,as HF, 0.02% tungsten, added as Na WO and 0.22% AMP) (average AlF Thesolution had a pH of 1.68, a total fluoride of 0.17% and when applied byspraying at F. for 7-15 seconds contact time to the same aluminumbuilder stock employed in Example I, produced coatings that wereapproximately the same weight range, had an average coating efficiencyof 0.25.

7. EXAMPLE m A 750 gallon tank for immersion coating was filled with asolution containing 0.5% CrO 0.25% fluoride, as H SiF 0.04% tungsten,added as Na WO 0.23% chromic nitrate and 0.22% AlF(F) (average AlF Thesolution had a pH of 1.55, a total fluoride of 0.4% and was raised intemperature to about 120 F. 3003 aluminum processed through the solutionby immersion for 30 seconds to 1 minute produced adherent, gold-coloredcoatings having an average weight of 40-60 milligrams per square ft. Atthis temperature the average coating efliciency was 0.25.

This solution was operated at varying temperatures by spraying portionsof the. solution on 3003 cleaned aluminum panels for a -second contacttime. At 100 F. the coating weight was 55 milligrams per square ft., at110 F. it was 62 milligrams per square ft., at 120 F. it was 84milligrams per square ft., at 135 F. it was 81 milligrams per squareft., at 150. F. it was 67 milligrams per square ft., and at 160 F. itwas 50 milligrams per square.

EXAMPLE IV A 2000 gallon solution was prepared containing 0.5 CrO 0.08%tungsten, added as Na WO 1.37% chromic nitrate, 0.02% fluoride as HF,and 1.87% Al(F) (average MP The solution had a pH of 1.71, atotalfluoride of 1.23% and was used in spray coating of 3003 aluminum at 120F. for 7-15 second contact time and produced adherent light-gold todark-gold coatings at coating weights averaging between about and about50 milligrams per square ft., with a coating eificiency average of 0.3.

EXAMPLE V A 2000 gallon solution was prepared containing 1.0% CrO 0.41%fluoride, as HBF 2.28% chromic nitrate, 0.3l%' tungsten, added as Na WOand 3.11% Al(F) average AlF This solution had a pH of 1.96, a totalfluoride of 2.51% and when operated in accordance with the proceduresand on the materials specified in Example I, for similar coatings, hadan average coating efficiency of 0.33.

EXAMPLE VI A solution was prepared containing 0.49% CrO 0.04% fluoride,as HF, 0.09% tungsten, added as Na WO and 0.236% Al(F) (average A11 Thesolution contained a total fluoride of 0.2 and had a pH of 168. Thesolution was heated to 120 F. and sprayed on 3003 aluminum panels for a15-second contact time to produce a uniform adherent coating having anaverage coating weight of 78 milligrams per square it. at a coatingefficiency of 0.19.

The above bath, when operated at 90 F., and under otherwise identicalconditions, was found to produce adherent coatings having an averageweight of 21 milligrams per square ft.; when operated at 100 F. producedcoatings having an average weight of 54 milligrams per square ft.; whenoperated at 130 F. produced coatings having an average weight of 98milligrams per square ft.; when operated at 140 F. produced coatingshaving an average weight of 98 milligrams per square it; when operatedat 150 F. produced coatings having an average weight of 86 milligramsper square ft.; and when operated at 160 F. produced coatings having anaverage weight of 76 milligrams per square ft.

To portions of the above bat-h, sodium hydroxide Was added sufiicient toproduce a pH of 1.81, and under the same conditions of application at120 F. and on similar panels, an average coating weight of 77 milligramsper square ft. was obtained at a coating efliciency, of 0.20. To anotherportion of the solution, additional sodium hydroxide was added toproduce a pH of 1.91 and when operated under similar conditions producedcoatings having an average weight of 71 milligrams per square ft. at acoating efficiency of 0.24. To another portion of the above solution,suflicienttsodium hydroxide was added to produce a pH of 2.12 and whenoperatedunder similar conditions was found. to produce adherent coatingshaving an average weight of 45 milligrams per square ft. at a coatingefli'ciency of 0.27. To another portion of the above solution,sutlieient sodium hydroxide was added to produce a pH of 2.28, and whenthis solution was oper ated, under similar conditions, adherent coatingswere produced having an averageweight of 22 milligrams per square ft. ata coating efliciency of 0.28.

Another solution was prepared containing 0.49% .CrO 0.09% tungsten, asNa WO 0.02% fluoride, as HF and 0.266% Al(F) (average AlF This solutionhad a pH of 1.70 and a total fluoride content of 0.20%. This solution,operating at F., was sprayedon 3003 aluminum panels for 15 secondscontact time and produced an average coating weight of. 37 milligramsper square ft. at a coating efiiciency of 0.28. 0.48% HNO was added tothis solution to convert the solution into one having a pH of 1.21. andwhen operated under similar conditions produced adherent coatings havingan average weight of l 66 milligrams per square it. at a coatingefficiency of 0.27. A single package replenishing material is suitablefor use in maintaining the operating solutions of this invention inoptimum coating-forming condition and a replenishing material for thispurpose may satisfactorily contain:

Parts by weight CrO 15-20 HF 7-12 HNO 1.4-7 Na2WO4'2H O Al(F) (averageAlF 0.6-1.6

CrO 17-19 HF 7-8 .HNO 1 4-1 6 N32W042H2O 3-5 A1(F) (average A1F 0.6-1.0H BO 1.2-2.0

An especially satisfactory replenishing material for use with acontinuous strip-line operation in which no cation exchange resin bed isemployed is the following:

Parts by weight cro 18-20 HNO 5-7 Na2WO42H2O 3-6 A1(F) (average A1F0.8-1.5. HF 8-11 A replenishing material particularly satisfactory inproduction operations in which the parts to be coated are moved througha spray installation on a monorail conveyor is the following:

Parts by weight CI'O3 HF 8-11 HNO 4.5-6.5 N21 WO 2H O 3 grams/liter CrOat least 0.1 gram per liter of the tungsten anion, about 2.2 to about 32grams per liter of an aluminum fluoride complex ion and sufficientfluoride ion to cause said solution to coat said aluminum surface.

2. A method for forming a protective coating on the surfaces of aluminumand aluminum alloys which comprises the step of applying to the surfacethereof an aqueous acidic solution consisting essentially of, in percentw./v., 0.05-1 CrO 0.16-2.7 fluoride ion, 0.01-0.4 tungsten and 022-32A1(F) complex.

3. A method for forming a protective coating on the surfaces of aluminumand aluminum alloys which comprises the step of applying to the surfacethereof an aqueous acidic solution consisting essentially of, in percentW./v., 0.2-0.5 CrO 0.16-1.6 fluoride ion, 0.03-0.1 tungsten and 0.22-1.9A1(F) complex.

4. A method for forming a protective coating on the surfaces of aluminumand aluminum alloys which comprises treating the surface thereof with anaqueous acidic solution consisting essentially of, in percent w./v.,0.05-1 CrO 0.16-2.7 fluoride ion, 0.01-0.4 tungsten and 022-32 A1(F)complex, said solution having a pH in the range of about 1.1 to 2.3.

5. A method in accordance with claim 4 wherein said pH is maintained insaid range by the periodic addition thereto of an acid in an amountequivalent to about 0.05% to about 3% of nitric acid.

6. A method for forming a protective coating on the surfaces of aluminumand its alloys which comprises the step of applying to said surface anaqueous acidic solution consisting essentially of, in percent W./v.,0.05-1 CrO 0.16-2.7 fluoride ion, 0.01-0.4 tungsten and 0.22-3.2 A1(F)complex, said solution having a pH in the range of about 1.1 to about2.3, periodically cycling a portion of said solution through a cationexchange resin bed to maintain said aluminum fluoride complex ion withinsaid range.

7. A method for forming a protective coating on the surfaces of aluminumand its alloys which comprises the step of applying to said surface anaqueous acidic solution consisting essentially of, in percent W./v.,0.2-0.5 CrO O.16-l.6 fluoride ion, 0.03-0.1 tungsten and 0.22-1.9 A1(F)complex, said solution having a pH in the range of about 1.1 to about2.3, periodically cycling a portion of said solution through a cationexchange resin bed to maintain said aluminum fluoride complex withinsaid range.

8. A method for forming a protective coating on the surfaces of aluminumand its alloys which comprises the step of applying to said surface anaqueous acidic solution consisting essentially of, in percentw./v.,0.05-1 CrO 0.16-1.6 fluoride ion, 0.03-0.1 tungsten and 0.2-3A1(F) complex ion, said solution having a pH in the range of about 1.6to about 2.1, periodically cycling a portion of said solution through acation exchange resin bed to maintain said aluminum fluoride complexwithin said range.

9. An aqueous acidic solution for forming a protective coating on thesurface of aluminum and aluminum alloys which consists essentially of anaqueous acidic solution containing about 0.5 to about 10 grams/liter CrOthe tungsten anion, an aluminum fluoride complex and at least about0.16% fluoride ion.

10. An aqueous acidic solution for forming a protective coating on thesurface of aluminum and aluminum alloys which consists essentially of anaqueous acidic solution containing, in percent w./v., 0.05-1 CrO0.16-2.7 fluoride ion, 0.0l-0.4 tungsten and 022-32 A1(F) complex.

11. An aqueous acidic solution for forming protective coating on thesurface of aluminum and aluminum alloys which consists essentially of anaqueous acidic solution containing, in percent w./v., 0.05-1 CrO0.16-2.7 fluoride ion, 0.01-0.4 tungsten and 0.22-3.2 (A1('F) complex,said solution having a pH in the range of 1.1 to 2.3.

12. An aqueous acidic solution for forming a protective coating on thesurface of aluminum and aluminum alloys which consists essentially of anaqueous acidic solution containing in percent w./v., 0.05-1 CrO 0.16-2.7fluoride ion, 0.0l-0.4 tungsten and 0.22-3.2 A1(F) complex, saidsolution having a pH in the range of 1.6 to about 2.1.

13. A replenishing material for forming solutions for coating aluminumand aluminum alloys, which consists esentially of in parts by weight:CrO 15-20; HF, 7-12; HNO 1.4-7; Na WO .2H O, 3-6.5; A1(F) (averageA1F3), 0.6-1.6.

14. A replenishing material for forming solutions for coating aluminumand aluminum alloys, which consists essentially of in parts by weight:CrO 17-19; HF, 7-8; HNO 1.4-1.6; Na WO 2H O, 3-5; A1(F) (average A1F0.6-1.0; H BO 1.2-2.0.

15. A replenishing material for forming solutions for coating aluminumand aluminum alloys, which consists essentially of in parts by Weight,CrO 18-20; HF, 10-12; HNO 5-7; Na WO 2H O, 3-6; A1(F) (average A1F0.8-1.5.

16. A replenishing material for forming solutions for coating aluminumand aluminum alloys, which consists essentially of in parts by weight:CrO 15-17; HF, 8-11; I-INO 4.5-6.5; Na WO 2H O, 3.5-6.5; A1(F) (averageA1F 1-1.6.

References Cited UNITED STATES PATENTS 2,276,353 3/ 1942 Thompson1486.27 X 2,868,679 1/ 1959 Pimbley 1486.2 2,967,791 1/ 1961 Halversen1486.16

ALFRED L. LEAVITT, Primary Examiner. RALPH S. KENDALL, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,347,713 October 17, 1967 Herman J. Lodeesen et a1.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 1, line 68, "acceptanble" should read acceptable Column 2, line38, "tne" should read the Column 5, line 38, "solution solution" shouldread solution line 46, "or aluminum" should read of aluminum Column 8,

line 58, "HF--8-ll" should read HF-10l2 Signed and sealed this 20th dayof January 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

1. A METHOD FOR FORMING A PROTECTIVE COATING ON THE SURFACES OF ALUMINUMAND ALUMINUM ALLOYS WHICH COMPRISES TREATING THE SURFACE THEREOF WITH ANAQUEOUS ACIDIC SOLUTION CONSISTING ESSENTIALLY OF ABOUT 0.5 TO ABOUT 10GRAMS/LITER CRO3, AT LEAST 0.1 GRAM PER LITER OF THE TUNGSTEN ANION,ABOUT 2.2 TO ABOUT 32 GRAMS PER LITER OF AN ALUMINUM FLUORIDE COMPLEXION AND SUFFICIENT FLUORIDE ION TO CAUSE SAID SOLUTION TO COAT SAIDALUMINUM SURFACE.